avi containers hold a (usually mpeg4) video stream and one or more audio streams (usually mp3).

There are two flavors of avi, interleaved and non-interleaved. The interleaved type consists of packets of video and audio "interleaved" to make synchronous playback and seeking possible. The non-interleaved type consists of the entire video stream followed by the entire audio stream, a fragile construction indeed. It is possible to concatenate avi container content and produce really horrible things, even to the extent of mixing interleaved and non-interleaved avi material.

The avi container format was invented by Micro$oft.

Somebody with a more mature understanding of this needs to edit this page!